1. Field of the Invention
The present invention relates to a trigger circuit for a bilateral switch of a 4-terminal thyristor capable of passing a continuous A.C. signal for use in a telephone exchange system or the like.
2. Description of the Prior Art
A semiconductor switch circuit comprising a 4-terminal thyristor capable of switching A.C. signals with a small trigger voltage under any load conditions is disclosed in the U.S. Pat. No. 3,959,668, in which a pair of 4-terminal thyristors each having a cathode gate and an anode gate are connected in inverse-parallel thereby to constitute a bilateral switch, and a cathode gate trigger circuit including a positive voltage supply for feeding a current into the respective cathode gates is connected to each of the cathode gates of the respective 4-terminal thyristors constituting the bilateral switch, while an anode gate trigger circuit including a negative voltage supply for taking out a current from the respective anode gates is connected to each of the anode gates of the 4-terminal thyristors, whereby gate current is caused to flow continuously with a small trigger voltage irrespective of the amplitude of the signal voltage so that the A.C. signal current can be passed through the thyristors without any momentary interruption even when the A.C. signal current is reduced below a self-holding current value of the thyristors. However, because the semiconductor bilateral switch circuit is driven with the P-type gate and the N-type gate, it requires the positive and negative power supplies and two switching circuits. Therefore, the semiconductor bilateral switch circuit is uneconomical.
The present inventors have previously proposed a semiconductor switch circuit as shown and disclosed in the U.S. Pat. No. 4,058,741 in which a pair of 4-terminal thyristors are connected in inverse-parallel to constitute a bilateral switch, and a current-supplying type constant-current circuit including a power supply for supplying a current and a current-sinking type constant-current circuit including no power supply and sinking a current are connected to each cathode gate and each anode gate, respectively, of the thyristors and a switching circuit is connected between the constant-current circuits so that it responds a potential of the bilateral switch to automatically feed the trigger current of the current-supplying type constant-current circuit into the cathode gates of the bilateral switch and also into the current-sinking type constant-current circuit, whereby the switch circuit is controlled by a single power supply and a single switch. However, since this semiconductor switch circuit feeds the trigger current of the current-supplying type constant-current circuit into the cathode gate of the bilateral switch and also into the current-sinking type constant-current circuit, when the potential of the bilateral switch and the reference potential of the current-sinking type constant-current circuit are equal, the trigger current of the current-supplying type constant-current circuit flows through the both circuits so that a current of I.sub.K /2 flows into each circuit, where I.sub.K is the constant current. Thus, in order to prevent the occurrence of a case where the current does not reach the firing current for the anode gate and the cathode gate of the bilateral switch, the constant current I.sub.K must be selected to more than twice as high as the cathode gate firing current. Furthermore, there is another drawback that in order to pass the A.C. signal through the semiconductor switch circuit by maintaining it in a closed position a circuit for holding the conducting state thereof must be separately provided.